Power supply for a cochlear implant

ABSTRACT

A power supply control system for use with a tissue stimulating prosthesis, such as a cochlear implant. The power supply control system comprises a first battery ( 31 ), a second battery ( 32 ), at least a third battery ( 33 ), and a switching system ( 36 ). The first and second batteries ( 31, 32 ) are electrically connected in series to provide power to the prosthesis, while the third battery ( 33 ) is electrically connectable through the switching means ( 36 ) in parallel with either the first battery ( 32 ). The third battery ( 33 ) is electrically connected by the control system in parallel with whichever one of said first battery ( 31 ) or said second battery ( 32 ) has lowest voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication Number PCT/AU02/00074, filed on Jan. 24, 2002, which claimspriority of Australian Patent Application Number PR 2693, filed Jan. 24,2001.

FIELD OF THE INVENTION

The present invention relates to a power supply for an implant, such asa cochlear implant, and in particular, to a power supply having aplurality of batteries and a means for controlling the use of thebatteries by the implant.

BACKGROUND ART

In many people who are profoundly deaf, the reason for deafness isabsence of, or destruction of, the hair cells in the cochlea whichtransduce acoustic signals into nerve impulses. These people are thusunable to derive suitable benefit from conventional hearing aid systems,no matter how loud the acoustic stimulus is made, because there isdamage to, or an absence of the mechanism for nerve impulses to begenerated from sound in the normal manner.

It is for this purpose that cochlear implant systems have beendeveloped. Such systems bypass the hair cells in the cochlea anddirectly deliver electrical stimulation to the auditory nerve fibres,thereby allowing the brain to perceive a hearing sensation resemblingthe natural hearing sensation normally delivered to the auditory nerve.U.S. Pat. No. 4,532,930, the contents of which are incorporated hereinby reference, provides a description of one type of traditional cochlearimplant system.

Cochlear implant systems have typically consisted of two essentialcomponents, an external component commonly referred to as a processorunit and an internal implanted component commonly referred to as astimulator/receiver unit. Traditionally, both of these components havecooperated together to provide the sound sensation to a user.

The external component has traditionally consisted of a microphone fordetecting sounds, such as speech and environmental sounds, a speechprocessor that converts the detected sounds into a coded signal, a powersource such as a battery, and an external transmitter coil.

The coded signal output by the speech processor is transmittedtranscutaneously to the implanted stimulator/receiver unit situatedwithin a recess of the temporal bone of the user. This transcutaneoustransmission occurs via the external transmitter coil, which ispositioned, to communicate with an implanted receiver coil provided withthe stimulator/receiver unit. This communication serves two essentialpurposes, firstly to transcutaneously transmit the coded sound signaland secondly to provide power to the implanted stimulator/receiver unit.Conventionally, this link has been in the form of a radio frequency (RF)link, but other such links have been proposed and implemented withvarying degrees of success.

The implanted stimulator/receiver unit traditionally includes a receivercoil that receives the coded signal and power from the externalprocessor component, and a stimulator that processes the coded signaland outputs a stimulation signal to an intracochlea electrode whichapplies the electrical stimulation directly to the auditory nerveproducing a hearing sensation corresponding to the original detectedsound. As such, the implanted stimulator/receiver device has been arelatively passive unit that has relied on the reception of both powerand data from the external unit to perform its required function.

Traditionally, the external componentry has been carried on the body ofthe user, such as in a pocket of the user's clothing, a belt pouch or ina harness, while the microphone has been mounted on a clip mountedbehind the ear or on the lapel of the user.

More recently, due in the main to improvements in technology, thephysical dimensions of the speech processor have been able to be reducedallowing for the external componentry to be housed in a small unitcapable of being worn behind the ear of the user. This unit allows themicrophone, power unit and the speech processor to be housed in a singleunit capable of being discretely worn behind the ear, with the externaltransmitter coil still positioned on the side of the user's head toallow for the transmission of the coded sound signal from the speechprocessor and power to the implanted stimulator unit.

The introduction of an external unit able to be positionedbehind-the-ear (BTE) provides the user with increased freedom notpreviously experienced with the more conventional body worn externalprocessor. A BTE unit does not require long cables connecting all of thecomponents together and does not require a separate battery pack, butprovides a single unit capable of being discretely worn behind the earof a cochlear implant user which offers the same functionality of thebody worn devices without the obvious restrictions that such devicesplace upon the user. Due to the obvious benefits such a device offers tothe user, it is important that with increasing use of such a componentthat the reliability of the device be at least the equivalent of theprevious body-worn devices. This is especially important with regard tothe power supply incorporated in such a BTE device, as whilst it is muchsmaller, the power supply needs to be sufficient to ensure that thepower demands of the implant are met, at least for an acceptable periodof time.

As described above, battery cells are typically housed in the externalcomponentry and provide the necessary power for the components of theimplant.

In conventional body worn devices, other than BTE devices, the issue ofensuring that the power supply is sufficient to meet the needs of theimplant is not of particular concern. This is due in the main to thefact that the size of the component is such that it can accommodate asubstantial number of cells and a battery pack can be further employedwith such a component. As this component is carried on the body in aharness or the like, the size of the component is not of greatimportance.

However, with the introduction and increased usage of BTE devices andthe desire to provide such devices that are small enough to fit behindthe ear of the user or to be discretely worn on the head of the user,the space requirements of the device lead to restrictions in the typeand dimension of power supply that can be utilised. Where previously thenumber of cells required to form the power supply of the device has beenrelatively unrestricted, such more discrete and compact BTE devices havelimited space to house the cells to be used to supply the power for theimplant. Where a single battery cell provides insufficient power for allof the components, it has been known to mount two batteries in serieswithin the external componentry.

One type of known battery cell used in cochlear implants and in implantsutilising BTE units in particular such as those provided by the presentapplicant, is the zinc air cell. Such cells have several practicaladvantages. They have a very high energy density and can supply adevice's requirements for a relatively long period of time relative totheir size and weight. They also have a relatively constant power outputthroughout most of their life, thereby reducing the risk of dangerousrapid discharge, such as shorting. Therefore, such cells have particularapplication to cochlear implants, which utilise these particularadvantages. As supplied, these cells do, however, occasionally sufferfrom a relatively high failure rate. Testing undertaken by the presentapplicant suggests that approximately 8% of supplied zinc air cells donot perform satisfactorily on delivery. Given that some cochlearimplants rely on two satisfactory cells being used in series, the chancethat a user will have a problem after replacing a pair of such batterycells increases to 15%. Such problems include finding that the cochlearimplant still does not work or stops working satisfactorily after arelatively short time following the insertion of new cells. Theseproblems can then result in the user incorrectly believing that theirdevice has failed and sending the device for repair or replacement, orfinding themselves unexpectedly losing their ability to experiencehearing sensation after they were sure that the power supply would lastfor a specific period of time.

When considering the amount of power that the external unit needs tosupply to the implant, it should be appreciated that this can vary quiteconsiderably from user to user. The amount of power required by theimplant depends on a number of factors. The stimulation rate and speechprocessing strategy employed by the user dictates greatly the powerrequirements of the implant. If the implant needs to stimulate at highrates then more power will be required, as will also be the case if acomplicated speech processing strategy is to be employed. Further tothis, the power requirements are strongly influenced by the thickness ofthe skin separating the external and internal coils in thetranscutaneous link. If this skin flap thickness is large, then theimplant will require more power to transmit across such a medium thanwould be the case if the skin flap thickness is quite small.

In any regard it is important that the external unit is designed suchthat there is sufficient power available for a wide range ofrequirements, from those users with large skin flap thicknesses and highrate stimulation strategies to those with small skin flap thicknessesand lower rate strategies. This ensures that an off-the-shelf device canbe supplied for all cases without the need for custom-made designsspecific to the particular user's power requirements.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

According to a first aspect, the present invention provides a powersupply for an electrically powered device, the power supply comprising afirst plurality of batteries, a switching means, and a second pluralityof batteries, wherein the first plurality of batteries are electricallyconnected in series and the second plurality of batteries areelectrically connectable through the switching means in parallel with atleast one of the first plurality of batteries.

According to a second aspect, the present invention provides a powersupply for an electrically powered device, the power supply comprising afirst battery, a second battery, at least a third battery, and aswitching means, the first and second battery being electricallyconnected in series and the at least third battery being electricallyconnectable through the switching means in parallel with either thefirst battery or the second battery.

In a preferred embodiment, the third battery is connected in parallel bythe switching means with whichever of the first and second batteries isexhibiting worse performance, which could be determined by which batteryhas the lower voltage. In this embodiment, the batteries will tend towork at substantially the same voltage and thus substantially evenlyshare the power load of the electrically powered device. Such anarrangement has the advantage that the probability of failure of a powersupply having three batteries in such an arrangement reduces to 2% andso the probability of a successful battery change increases to 98%,based on batteries having an 8% probability of being faulty.

The switching means preferably comprises an analog changeover switch,operable to connect the third battery in parallel with the first batteryor the second battery, and which may also be operable to disconnect thethird battery from both the first and second batteries. A low powercomparator can be used to compare the mid-point from a voltage divider.When the mid-point of the batteries indicates a mismatch, the powersupply preferably operates the switch to connect the third battery inparallel with whichever of the first or second batteries has the lowestvoltage. A small amount of hysteresis (eg. about 4 mV) can be providedto avoid excessive switching. The switching rate is preferably limitedto below about 50 kHz, more preferably, of the order of 20 kHz.

In the above embodiment, because all of the batteries are operating atsubstantially the same voltage (preferably within 8 mV), two of thebatteries are effectively in parallel. As switching between thebatteries occurs, only relatively small surge currents are preferablygenerated.

In one embodiment, the batteries of the power supply can berechargeable.

According to a third aspect, the present invention provides a powersupply control system for use with a tissue stimulating prosthesis, thepower supply control system comprising a first plurality of batteries, aswitching means and a second plurality of batteries, the first pluralityof batteries being electrically connected in series to provide power tothe prosthesis, and at least one of the second plurality of batteriesbeing electrically connectable through the switching means in parallelwith at least one of the first plurality of batteries; wherein the atleast one of the second plurality of batteries is electrically connectedby the control system in parallel with whichever one of said firstplurality of batteries has the lowest voltage.

According to a fourth aspect, the present invention is a power supplycontrol system for use with a tissue stimulating prosthesis, the powersupply control system comprising a first battery, a second battery, atleast a third battery, and a switching means, the first and secondbattery being electrically connected in series to provide power to theprosthesis, and the at least third battery being electricallyconnectable through the switching means in parallel with either thefirst battery or the second battery; wherein the at least third batteryis electrically connected by the control system in parallel withwhichever one of said first battery or said second battery has thelowest voltage.

According to a fifth aspect, the present invention provides a powersupply control system for use with a tissue stimulating prosthesis, thepower supply control system comprising a first plurality of batteries, asecond plurality of batteries, and a switching means, the firstplurality of batteries being electrically connected in series to providepower to the prosthesis, and at least one of the second plurality ofbatteries being electrically connectable through the switching means inparallel with at least one of the first plurality of batteries; whereinthe at least one of the second plurality of batteries is electricallyconnected by the control system in parallel with one of said firstplurality of batteries following detection by the control system thatthe voltage of said one of said first plurality of batteries is below apredetermined threshold.

The predetermined threshold may be determined by reference to a voltageof another of the first plurality of batteries.

According to a sixth aspect, the present invention provides a powersupply control system for use with a tissue stimulating prosthesis, thepower supply control system comprising a first battery, a secondbattery, at least a third battery, and a switching means, the first andsecond battery being electrically connected in series to provide powerto the prosthesis, and the at least third battery being electricallyconnectable through the switching means in parallel with either thefirst battery or the second battery; wherein the at least third batteryis electrically connected by the control system in parallel with one ofsaid first battery or said second battery following detection by thecontrol system that the voltage of said first battery or said secondbattery is below a predetermined threshold.

According to a seventh aspect, the present invention provides a powersupply control system for use with a tissue stimulating prosthesis, thepower supply control system comprising:

a first battery;

a second battery connected in series with the first battery to providepower to the prosthesis; and

a third battery being electrically connectable via switching means inparallel with either the first battery or the second battery; whereinthe control system controls the switching means to electrically connectthe third battery in parallel with said first battery or said secondbattery based on a determination by the control system that theoperation of said first battery or said second battery is below apredetermined threshold.

In these aspects, the batteries will tend to work at substantially thesame voltage and thus share the power load of the prosthesis. Such anarrangement has the advantage that the probability of failure of a powersupply having three batteries reduces to 2% and so the probability of asuccessful battery change increases to 98%. Such an arrangement alsoensures that should the system power requirements be too great for thetwo batteries in series alone, the third battery will be electricallyconnected to cover any increased power requirements.

The switching means of the present invention preferably comprises ananalog changeover switch. In preferred embodiments of the seventh aspectof the present invention, a low power comparator may be used to make thedetermination of satisfactory operation of the first and secondbatteries, for example by being used to compare the mid-point from avoltage divider positioned across the first and second batteries. Such acomparator may also be used in voltage measurements of the batteries inaccordance with the first to sixth aspects of the present invention. Insuch embodiments, when the mid-point of the batteries indicates amismatch, the comparator preferably operates the switch to connect thethird battery in parallel with whichever of the first or secondbatteries has the lowest voltage. As the third battery willapproximately halve the demand on the battery with which it is connectedin parallel, the other of the first and second batteries will be drainedat a faster rate. Hence, it is expected that it will repeatedly benecessary to connect the third battery across the other of the first andsecond batteries. The control system preferably causes such switching ofthe third battery between the first and second batteries to occur basedon the voltages measured by the voltage divider. In this regard, a smallamount of hysteresis (eg. about 4 mV) is preferably provided to avoidexcessive switching. The switching rate is preferably limited to belowabout 50 kHz. Such regular switching of the third battery between thefirst and second batteries will assist in ensuring the first and secondbatteries are drained at a similar rate.

In one embodiment, the tissue-stimulating prosthesis can comprise acochlear implant. The cochlear implant can comprise an externallymounted device that includes a speech processor.

For the purposes of the description provided below, reference will bemade to the prosthesis in the form of a cochlear implant. It is to beappreciated that the following description could apply, with appropriatemodification, to other systems adapted for implantation into bodytissue.

When in use, the power supply control system preferably controls thepower supply for the microphone, speech processor, electrode array andany other electrical or electronic componentry of the cochlear implantsystem. The power supply control system preferably ensures that not onlyis the power load shared with all of the batteries of the system butalso that should the system power requirements be large, such as incases of large skin flap thickness and high stimulation rate, then thepower supply is able to meet such needs.

In one embodiment, the power supply can be mounted within a case thatalso encloses the componentry of the electrical equipment, such as theprocessor means of the cochlear implant. In another embodiment, thepower supply can be mounted within a separate case with electricalconnection provided between the batteries and the componentry, such asthe processor means.

The first, second, and at least the third battery can each comprise azinc-air cell. It will, however, be appreciated that any suitablebattery cell could be utilised in the present invention. Each of thebatteries are also preferably surrounded by an electrically insulatingmaterial such that the batteries are electrically insulated from thecase in which they are mounted.

A battery charging means can be used to recharge the batteries of thepower supply.

In accordance with an eighth aspect, the present invention provides amethod of operating a power supply, the method comprising the steps of

electrically connecting a first battery and a second battery in series;and

electrically connecting a third battery in parallel with whicheverbattery of the first battery and the second battery exhibits worseperformance.

In accordance with a ninth aspect, the present invention provides amethod of operating a power supply for a tissue-stimulating prosthesis,the method comprising the steps of

electrically connecting a first battery and a second battery in series;and

electrically connecting a third battery in parallel with whicheverbattery of the first battery and the second battery exhibits worseperformance.

The methods of the eighth and ninth aspects of the invention preferablyfurther comprise the step of measuring the performance of the firstbattery and the second battery.

The methods of the eighth and ninth aspects of the invention preferablyfurther comprise, prior to the step of measuring the performance of thefirst battery and the second battery, disconnecting the third batteryfrom a parallel connection with either of the first battery or thesecond battery.

Such a step ensures that the measurement of performance of the firstbattery and the second battery, such as by measuring the voltage acrosseach of the first and second batteries, is carried out without theperformance of the third battery affecting the measurement.

Preferred embodiments of the present invention provide a system designedto maximise the performance of devices such as cochlear implants andother devices that are reliant upon installed battery power, in thepresence of an unreliable power supply. Preferred embodiments of theinvention may also provide a system designed to cater for the powerrequirements of a wide range of cochlear implant or tissue-stimulatingimplant users with varying system requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the invention is nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a pictorial representation of a typical cochlear implantsystem incorporating the present invention;

FIG. 2 is a circuit layout of a power supply for use in electricallypowering a cochlear implant;

FIG. 3 is a side elevational view of another embodiment of the externalcomponent of a cochlear implant having the power supply control systemof the present invention;

FIG. 4 is a side elevational view of the external component of thecochlear implant of FIG. 3 with the battery compartment cover removed;

FIGS. 5 to 7 illustrate typical limiting currents of various cells;

FIG. 8 illustrates the limiting current over time exhibited by a 3 cellembodiment of the present invention and by a prior art 2 cellarrangement, when loaded by a constant 2.2 Volt load;

FIG. 9 illustrates the limiting current vs mA hours for the 3 cellembodiment of the present invention and the 2 cell prior art arrangementreferred to in FIG. 8; and

FIG. 10 illustrates the duty cycle of a battery of the presentembodiment of the invention.

PREFERRED MODE OF CARRYING OUT THE INVENTION

An example of a device powered by the power supply system of the presentinvention can be seen in FIG. 1.

One embodiment of a cochlear implant utilising the present invention isdepicted in FIG. 1 and consists of two main components, namely anexternal component 10 including a speech processor 29, and an internalcomponent including an implanted receiver and stimulator unit 22.

The external component 10 includes an on-board microphone 27. The caseof the external component 10 is constructed and arranged so that it cansit on the outer ear 11 of the implantee. The case of the externalcomponent is also constructed so that it contains a power supply inaccordance with the present invention. The power supply provides thepower for the entire implant system.

A cable 13 extends from the case of the external component 10 to anexternal transmitter coil 24 which transmits electrical signals to theimplanted unit 22 via a radio frequency (RF) Link.

The implanted component includes a receiver coil 23 for receiving powerand data transmitted from the transmitter coil 24. A cable 21 alsoextends from the implanted receiver and stimulator unit 22 to thecochlear 12 and terminates in an electrode array 20. The signals thusreceived are applied by the array 20 to the basilar membrane 8 therebystimulating an auditory nerve 9. The operation of such a device isdescribed, for example, in U.S. Pat. No. 4,532,930.

Such a cochlear implant system as described in FIG. 1 requires a powersupply capable of being incorporated in the relatively small case of theexternal component 10 so that it can be positioned behind the ear of theimplant user. Further, the voltage of the power supply of this systemalso needs to be sufficient to provide a reliable supply of power to thesystem even in cases where the user has a large skin flap thicknessbetween transmitter coil 24 and receiver coil 23 resulting in increasedpower requirements to transmit the applicable data/power to theimplanted unit 22. The present invention provides a reliable solution tothese requirements.

One example of a circuit layout for a power supply that is housed in thecase of the external component 10 for powering a cochlear implant isdepicted generally as 30 in FIG. 2.

The power supply circuit 30 includes first battery 31 and a secondbattery 32 electrically connected in series. The output of the powersupply is provided at terminals 34 and can be switched on or off usingthe On/Off switch 35.

The power supply circuit 30 further includes at least a third battery 33that is electrically connectable in parallel with either the firstbattery 31 or the second battery 32 through an analog changeover switch36.

In the depicted embodiment, the third battery 33 is connected inparallel by switch 36 with whichever of the first and second batteries31,32 has the lowest voltage. In this embodiment, the batteries 31,32,33all tend to work at substantially the same voltage and thussubstantially evenly share the power load of the cochlear implantpowered by the power supply circuit 30.

Such an arrangement has the advantage that the probability of failure ofa power supply having three batteries in such an arrangement reduces to2% and so the probability of a successful battery change increases to98%, based on batteries having an 8% probability of being faulty at thetime of supply as tests by the applicant have revealed.

In the depicted embodiment, a comparator 37 is used to compare themid-point from a voltage divider. When the mid-point of the first andsecond batteries 31,32 indicates a mismatch, the comparator 37 operatesthe switch 36 to connect the third battery 33 in parallel with whicheverof the first or second batteries 31,32 has the lowest voltage. A smallamount of hysteresis (eg. about 4 mV) is built into the comparator toavoid excessive switching of switch 36. In the depicted embodiment, theswitching rate is of the order of about 20 kHz.

In the above embodiment, because all of the batteries 31, 32, 33 areoperating at the same voltage (within 8 mV), the batteries areeffectively in parallel. As switching between the batteries occurs, onlyrelatively small surge currents are generated since the small voltagedifference (8 mV) across the internal resistance (perhaps 20 ohms)amounts to only around 0.4 mA.

A prototype of the circuit shown in FIG. 2 has been tested, anddisplayed efficiency extremely close to 100%. The estimated losses atfull load of the circuit include a 400 μW series loss in the switch “on”resistances, a 13 μW switching loss in the comparator, a 13 μW loss dueto driving of switch capacitances, and a 15 μW resistive loss in thevoltage divider. Peak efficiency occurs at an output power of 15 mWgiving 99.5% efficiency, while efficiency of 99% is displayed at 2.5 mWand 44 mW output power. Ripple and noise levels were acceptable.

As described above, one example of a device that can be powered by thepower supply 30 is a tissue-stimulating cochlear implant prosthesis thathas components adapted for implant in an implantee's body. Anotherembodiment of an external component for such an implant is depicted as40 in FIGS. 3 and 4. The external component 40 houses a speechprocessor, and, has an in-built microphone 27.

The component 40 has a removable cover 41 enclosing a batterycompartment 42. FIG. 4 depicts the component 40 with the cover 41removed. Housed within compartment 42 are the first and second batteries31,32 connected in series, and third battery 33.

When in use, the batteries 31, 32, 33 provide power for all componentsof the cochlear implant including the microphone 27, a speech processor,the implanted electrode array, and any other electrical or electroniccomponentry of the cochlear implant whether it be external or internalof the body of the implantee.

While the batteries can be mounted within the case that also enclosesthe other componentry of the external component 40, the batteries couldbe mounted within a separate case with electrical connection providedbetween the batteries and the componentry of the implant, such as thespeech processor.

The depicted batteries 31,32,33 each comprise a zinc-air cell. It will,however, be appreciated that any suitable battery cell could be utilisedin the present invention. When mounted in a prosthesis, each of thebatteries 31,32,33 are also preferably surrounded by an electricallyinsulating material such that the batteries are electrically insulatedfrom the case in which they are mounted.

The batteries are preferably all of the same design, however the presentinvention may be applied to batteries having some differences in design.The present invention may also be applied to the use of rechargeablebatteries.

FIG. 5 illustrates typical limiting currents of each of four 675 sizezinc-air Activair HPX battery cells, presented as limiting current in mAvs time. FIG. 6 illustrates typical limiting currents of each of seven675 size zinc-air Varta V675 battery cells, again presented as limitingcurrent in mA vs time. FIG. 7 illustrates the performance of six pairsof Rayovac 675 size zinc-air cells, presented as limiting current in mAvs time. As can be seen from FIGS. 5–7, the reliability of such cells isrelatively poor, with the performance from one cell to the next beingrelatively inconsistent.

FIG. 8 illustrates the limiting current over time exhibited by a 3 cellembodiment of the present invention compared with the limiting currentover time of three prior art 2 cell arrangements, when loaded by aconstant 2.2 Volt load. As can be seen, the 3 cell arrangement of thepresent invention significantly improves the limiting current of thepower supply, such that any given load current can be supplied for alonger time by the present embodiment of the invention than the priorart two cell arrangement. This is better shown in FIG. 9, whichillustrates the limiting current vs mA hours for the 3 cell embodimentof the present invention and the 2 cell prior art arrangement. As can beseen, for a load current of, say, 15 mA, the available capacity hasincreased from 268 mA hours to 536 mA hours, exactly double. Dischargingthe cells to exhaustion revealed a capacity increase from 350 mAh forthe prior art 2 cell arrangement to 572 mAh for the present embodimentof the invention. Hence, the present embodiment of the inventionexhibits a 63% greater capacity than the 2 cell prior art arrangement,which is a better improvement than the 50% expected improvement. Thismay be due to a increased capacity of the batteries when loaded lessheavily.

Finally, to demonstrate the action of the third battery cell, thepercentage of time that one of the two series cells was connected inparallel with the third cell was recorded over time. The results aredisplayed in FIG. 10. FIG. 10 shows that, initially, the first of theseries cells carried more current for much of the time than the second.However, from 24 hours to 35 hours the situation was reversed and theother cell carried more current. This demonstrates the ability of thesystem to adapt to the randomly varying output of the cells andimbalances between the cells.

While the illustrated and described embodiment comprises two batteriesplaced in series and a third battery being electrically connectablethrough a switching means in parallel with either of the two batteriesplaced in series, it is envisaged that more than three batteries couldbe used, employing more than two batteries in series and more than onebattery electrically connectable in parallel with one or more of theseries-connected batteries without going beyond the scope of the presentinvention.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A power supply for an electrically powered device comprising: a firstbattery; a second battery; at least a third battery; and a switchingmeans; wherein, the first and second battery being electricallyconnected in series, and the at least a third battery being electricallyconnected by the switching means in parallel with one of the firstbattery or the second battery which has a lower voltage, when the lowervoltage is below the voltage of the other one of the first and secondbatteries by a predetermined value.
 2. The power supply of claim 1wherein the switching means comprises an analog changeover switch. 3.The power supply of claim 1 further comprising a comparator thatcompares a mid-point voltage from a voltage divider with thepredetermined value, such that when a mid-point voltage of the batteriesindicates a mismatch with the predetermined value, the power supplyoperates the switching means to connect the third battery in parallelwith the one of the first or second batteries which has the lowervoltage.
 4. The power supply of claim 3 wherein the comparator is a lowpower comparator.
 5. The power supply of claim 1 wherein thepredetermined value is at least 4 mV.
 6. The power supply of claim 1wherein at least one of the batteries of the power supply isrechargeable.
 7. A power supply control system for use with a tissuestimulating prosthesis comprising: a first battery; a second battery; atleast a third battery; and a switching means wherein, the first andsecond battery being electrically connected in series to provide powerto the prosthesis, and the at least a third battery being electricallyconnected by the switching means in parallel with one of the firstbattery or the second battery which has a lower voltage, when the lowervoltage is below the voltage of the other one of the first and secondbatteries by a predetermined value.
 8. The power supply control systemof claim 7 wherein the switching means comprises an analog changeoverswitch.
 9. The power supply control system of claim 7 further comprisinga comparator that compares a mid-point voltage from a voltage dividerpositioned across the first and second batteries with the predeterminedvalue, such that when a mid-point voltage of the batteries indicates amismatch, the power supply control system operates the switching meansto connect the third battery in parallel with the one of the first orsecond batteries which has the lower voltage.
 10. The power supplycontrol system of claim 7, wherein the predetermined value is at least 4mV.
 11. The power supply control system of claim 7 wherein thetissue-stimulating prosthesis comprises a cochlear implant.
 12. Thepower supply control system of claim 11 wherein the cochlear implantcomprises an external component that houses a speech processor.
 13. Thepower supply control system of claim 12 wherein the external componenthas a case, the power supply being mounted within the case.
 14. Thepower supply control system of claim 7 wherein the first, second, and atleast the third battery each comprise a zinc-air cell.
 15. A powersupply control system for use with a tissue stimulating prosthesiscomprising: a first battery; a second battery; at least a third battery;and a switching means wherein, the first and second battery beingelectrically connected in series to provide power to the prosthesis, andthe at least a third battery being electrically connected by theswitching means in parallel with either one of the first battery or thesecond battery which has a lower voltage, when the lower voltage isbelow the voltage of the other one of the first and second batteries bya predetermined value.
 16. The power supply control system of claim 15wherein the switching means comprises an analog changeover switch. 17.The power supply control system of claim 15 further comprising acomparator that compares a mid-point voltage from a voltage dividerpositioned across the first and second batteries with the predeterminedvalue, such that when a mid-point voltage of the batteries indicates amismatch, the power supply control system operates the switching meansto connect the third battery in parallel with the one of the first orsecond batteries which has the lower voltage.
 18. The power supplycontrol system of claim 15 wherein the predetermined value 4 mV.
 19. Thepower supply control system of claim 15 wherein the tissue-stimulatingprosthesis comprises a cochlear implant.
 20. The power supply controlsystem of claim 19 wherein the cochlear implant comprises an externalcomponent that houses a speech processor.
 21. The power supply controlsystem of claim 20 wherein the external component has a case, the powersupply being mounted within the case.
 22. The power supply controlsystem of claim 15 wherein the first, second, and at least the thirdbattery each comprise a zinc-air cell.
 23. A power supply control systemfor use with a tissue stimulating prosthesis, the power supply controlsystem comprising: a first battery; a second battery connected in serieswith the first battery to provide power to the prosthesis; and a thirdbattery being electrically in parallel with one of the first battery orthe second battery by a switching means; wherein the control systemcontrols the switching means to electrically connect the third batteryin parallel with said one of the first battery or the second batterybased on a determination by the control system that the operation ofsaid first battery or said second battery is below a predeterminedvoltage value and wherein the switching means operates only when thevoltage of the battery of the first and second batteries having a lowervoltage is below the voltage of the other battery of the first andsecond batteries by the predetermined voltage value.
 24. The powersupply control system of claim 23 wherein the control system is operableto measure a voltage of the first battery and a voltage of the secondbattery in order to make the determination of which of said firstbattery and said second battery are below the predetermined voltagevalue.
 25. The power supply control system of claim 23 wherein theswitching means comprises an analog changeover switch.
 26. The powersupply control system of claim 23 further comprising a comparator thatcompares a mid-point voltage from a voltage divider positioned acrossthe first and second batteries with the predetermined voltage value,such that when a mid-point voltage of the batteries indicates amismatch, the power supply control system operates the switching meansto connect the third battery in parallel with the one of the first orsecond batteries which has the lower voltage.
 27. The power supplycontrol system of claim 23 wherein the predetermined voltage is at least4 mV.
 28. The power supply control system of claim 23 wherein thetissue-stimulating prosthesis comprises a cochlear implant.
 29. Thepower supply control system of claim 28 wherein the cochlear implantcomprises an external component that houses a speech processor.
 30. Thepower supply control system of claim 29 wherein the external componenthas a case, the power supply being mounted within the case.
 31. Thepower supply control system of claim 23 wherein the first, second, andat least the third battery each comprise a zinc-air cell.